This invention relates to a ribbon drive mechanism, especially for high speed impact printers and more particularly to a crown roller structure used in a high speed impact printer.
High speed printers of the type with which the present invention is primarily concerned print "on the fly" at relatively high speeds. More specifically, in one class of printers, the character or font dies are mounted on a continuously moving endless carrier that is drawn past an aligned array of print hammers. Positioned adjacent the dies is the medium (paper) on which the characters are to be printed, and interposed between the medium and the dies is an ink-impregnated ribbon that is continuously moved. In operation, when a print hammer is activated, it causes a font die to impact upon the paper with the ribbon in between. This causes the character on the die to be printed. Hereinafter the die and the associated print hammer will be referred to as the print head.
Because of the high speed nature of such printers, incremental movement of the ribbon is precluded. Further, the ribbon advance mechanism is required to be of the reversing type and should operate relatively maintenance free. One appropriate ribbon advance mechanism meeting these objectives is described in a patent issued to Arthur F. Riley, U.S. Pat. No. 3,825,103, assigned to the assignee hereof, entitled "High Speed Printer Having Improved Ribbon Driving, Reversing and Tensioning Mechanism".
In order to operate economically, it is obvious that the ink on the ribbon should be utilized as completely as possible before the ribbon is replaced. As a result, it is advantageous for the entire width as well as the entire length of the ribbon to be exposed to impacting by the print head. It is also necessary for the ribbon to remain parallel to the printing medium (paper). However, it has been found in using printers such as the one described in the above-mentioned Riley patent that under certain conditions uneven ribbon wear at one edge results in elongation of the fibers causing the ribbon to rub against the flange of one or more ribbon guide rollers and eventually rollover, forming a double layer of ribbon adjacent the paper. Accordingly, the print head can impact upon the double layer, no ribbon at all, or a combination of folded ribbon and no ribbon, depending upon the relative positions of the folded ribbon and the print head. In any of these cases the printing is distorted.
It is therefore a principal object of the invention to provide a ribbon advance mechanism to prevent this rollover which distorts and destroys the printing capability.
It is also an object of the present invention to insure that the maximum area (width) of the ribbon be used for printing.
It is a further object to provide a ribbon advance mechanism which is not adversely affected by the wear or elongation imparted to the ribbon by the repeated impact of the print head.